1. Field of the Invention
The invention relates to a mobile radio system comprising fixed stations and mobile stations, in which system an allocation of radio channels is assigned to each fixed station and, while the system is in the operating mode, the quality of the radio channel in use is measured.
The invention likewise relates to a fixed station and an evaluation means respectively, for such a radio system.
2. Description of the Related Art
Mobile radio systems for covering rather large coverage areas have been structured as cellular radio networks for a rather long time. Each cell of such a radio network comprises at least one fixed station which provides radio connection to the mobile stations located in its radio cell. Such a radio system is, for example, the Pan-European GSM system which is being established. A survey of the GSM system is found, for example, in "An Overview of the GSM System", Bernard J T. Mallinder, Conference Proceedings, Digital Cellular Radio Conference, Oct. 12-14, 1988, Hagen, Federal Republic of Germany, pages 1a/1-1a/13.
Based on a preceding radio network planning, a channel allocation plan was established in the GSM system, which makes an allocation of radio channels available as a radio channel list to each individual fixed station. In what is commonly referred to as Operations and Maintenance Centres (OMCs), the radio channel lists of the fixed stations allocated to the individual OMCs can be entered by means of a computer terminal by an operator. On the basis of the radio channel list thus assigned to each fixed station, a free radio channel is chosen to establish and maintain a radio connection.
To ensure undisturbed operation during a telephone connection, the GSM system further has the facility to move to another frequency within a cell (intracell handover) and also to move to another cell (intercell handover). To judge its radio situation, a mobile station continuously measures the signal strength of adjacent fixed stations and the signal strength as well as the bit error rate of the radio channel in use. These measured values are sent to the fixed station to which that radio channel is use. There is then decided on the basis of the received measured values whether a change of radio channel or a change of radio cell is necessary.
For making the frequency band which is available for the relevant radio system, accessible to a maximum number of users in the whole mobile radio system, great importance is attached to the radio network planning.
In the radio network planning each fixed station is assigned a limited allocation of radio channels from the total number of available radio channels. In adjacent radio cells mutually different radio channels are used, so that mutually interfering radio channels can be avoided. However due to the limited frequency band, the radio channels in the whole radio network, must to be re-used as often as possible to obtain maximum user capacity.
The number of channels necessary for a fixed station is determined by the size of the radio cell and the local traffic load to be expected. For example, more radio channels may be necessary in the coverage area of a large city than in an entirely rural area. Partly for this reason, there are already radio cells of variable size which make it possible in so-called small cells to re-use the sound radio channel at a rather small distance from the small cell when the transmitter power is diminished accordingly.
The distance these radio cells are to be kept apart to avoid radio interference depends on many factors. These factors are, for example, the transmitter power and the conditions of radio wave propagation caused by shadows, bends, scatterings, reflections and multipath propagation on the radio transmission path.
When radio cells are planned, topographical data (such as, differences in height, buildings and so on) are taken into consideration by means of statistical methods.
Published PCT application WO 90/10342 discloses a method of radio cell planning, according to which field strength and interference measurements are made in the planning phase with instrument cars especially made for this purpose, the results being stored in a data bank. To each radio cell are assigned a number of radio channels which correspond to the estimated traffic load to be expected. On the basis of the measurement results a what is commonly referred to as exclusion matrix is then computed, which reflects the interaction between the individual radio cells. Using an iterative assignment algorithm the radio channel assignment is effected in random order. If no complete channel assignment can be effected in this manner, a new attempt is made in any one of said steps.
Even if such a frequency planning is carried out very carefully, it cannot always be achieved that only on the basis of the planning, even if this planning is supported by measurements, there will be a really optimum frequency distribution in the radio network.